1. Field of the Invention
The present invention relates to a wafer suction method, a wafer suction stage, and a wafer suction system used, e.g., when measuring the electrical characteristics of a wafer.
2. Background Art
Japanese Laid-Open Patent Publication No. 2003-347335 discloses a method wherein a porous metal section is provided in the surface of a stage and a wafer is mounted on this porous metal section. The lower surface of the porous metal portion adjoins a hollow space in the stage. Air is evacuated from this hollow space so that the wafer is attached to the upper surface of the porous metal section by suction.
Japanese Laid-Open Patent Publication No. 2008-4739 discloses a method wherein the electrical characteristics of a wafer are measured using the stage as an electrode. Japanese Laid-Open Patent Publication No. 2011-49337 discloses a method wherein a conductive sheet is formed on the bottom electrode of a wafer so as to prevent the bottom electrode of the wafer from being damaged by foreign matter, etc. caught between the bottom electrode and the stage.
The technique disclosed in the above-cited Publication No. 2003-347335 is disadvantageous in that portions of the porous metal section through which air is evacuated at a higher rate by the evacuation pump apply a higher suction pressure to the wafer than portions of the porous metal section through which air is evacuated at a lower rate. This means that powerful evacuation force is required to apply adequate suction pressure to the entire bottom surface of the wafer. The use of such evacuation force, however, results in deformation of the wafer. This problem is especially significant with thin wafers.
The deformation of the wafer occurring when the wafer is attached to the stage by high suction pressure is caused, e.g., by foreign matter caught between the stage and the wafer. In order to prevent deformation of the wafer due to the presence of such foreign matter, it is necessary to attach a conductive sheet to the bottom surface of the wafer. This, however, results in increased cost.
One method for avoiding this problem is to reduce the evacuation pressure (or vacuum pressure) of the evacuation pump, which, however, reduces the degree of adhesion between the wafer and the stage. A reduction in the degree of adhesion between the wafer and the stage causes problems such as increased contact resistance between them when the stage is used as an electrode.